Development of piezoelectric transformer-coupled solid state relays for electrical circuit control in railway systems

A solid state relay (SSR) is an electrical ON-OFF control device through which the delivery of electrical power to loads is switched either ON or OFF by a small control signal. The device serves the same function as a classical electromechanical relay (EMR) but carries no mechanical switching mechanism so that it has a faster switching time, a lower switching noise, a stronger tolerance to mechanical shock and vibration, a longer lifetime, a smaller size and a lighter weight. Photo-coupled SSRs (PcSSRs) are regarded as the most popular type of SSR nowadays. However, the susceptibility of PcSSRs to power surge and lightening often requires specific protections to ensure their normal operation and performance, especially for railway systems. To advance SSR technology for an improved electrical circuit control in railway systems, we have developed a novel type of SSR, called piezoelectric transformer-coupled SSRs (PTcSSRs), by utilising the high input-output electrical isolation property of a modified Rosen-type piezoelectric transformer (PT) for coupling between the input low-voltage dc-ac control circuit and the output high-voltage ac load circuit. In this paper, we report the design, development and performance of a PTcSSR having a 12 V dc input and a 40-240 V, 2 A, 50/60 Hz ac output, and compare the measured performance data with those of a commercial PcSSR used for electrical circuit control in modern railway systems. The development of the PT and its input-output circuit is also discussed.